1. Field of the Invention
This invention pertains generally to solid-state electrochemical devices, and more particularly to an electrochemical device support structure and an illustrative solid oxide fuel cell with exceptional electrochemical performance.
2. Description of Related Art
Solid-state electrochemical devices are becoming increasingly important for a variety of applications including energy generation, oxygen separation, hydrogen separation, coal gasification, and the selective oxidation of hydrocarbons. Fuel cells, for example, hold the promise of an efficient, combustion-less, low pollution technology for generating electricity. Because there is no combustion of fuel involved in the process, fuel cells do not create any of the pollutants that are commonly produced in conventional electricity generation by boilers or furnaces and steam driven turbines. Indeed, water, heat and electricity are the only products of fuel cell systems designed to use hydrogen gas.
Solid-state electrochemical devices are normally cells that include two porous electrodes, the anode and the cathode, and a dense solid electrolyte membrane disposed between the electrodes. In the case of a typical solid oxide fuel cell (SOFC), the anode is exposed to fuel and the cathode is exposed to an oxidant in separate closed systems to avoid any mixing of the fuel and oxidants due to the exothermic reactions that can take place with hydrogen fuel. The electrolyte membrane is normally composed of a ceramic oxygen ion conductor in solid oxide fuel cell applications. The porous anode may be a layer of a ceramic, a metal or, most commonly, a ceramic-metal composite (“cermet”) that is in contact with the electrolyte membrane on the fuel side of the cell. The porous cathode is typically a layer of a mixed ionically and electronically-conductive (MIEC) metal oxide on the oxidant side of the cell.
In implementations such as fuel cells and oxygen and syn-gas (H2+CO) generators, the solid membrane is normally an electrolyte composed of a material capable of conducting an ionic species, such as oxygen or hydrogen ions, yet has a low electronic conductivity. In other implementations, such as gas separation devices, the solid membrane is composed of a mixed ionic electronic conducting material (“MIEC”). In either case, the electrolyte/membrane must be dense and pinhole free (“gas-tight”) to prevent mixing of the electrochemical reactants. In all of these devices a lower total internal resistance of the cell improves performance.
Electricity is generated in a fuel cell through the electrochemical reaction that occurs between a fuel (typically hydrogen produced from reformed methane) and an oxidant (typically air). This net electrochemical reaction involves charge transfer steps that occur at the interface between the ionically-conductive electrolyte membrane, the electronically-conductive electrode and the vapor phase of the fuel or oxygen.
For example, in the operation of a typical solid oxide fuel cell, a fuel such as hydrogen, methane or carbon monoxide is cycled through the anode side of the cell and the fuel reacts with oxide ions (O−2) from the electrolyte. Examples of possible reactions at the anode that can occur in this setting, depending on the fuel used, are as follows:H2+O−2→H2O+2e−CO+O−2→CO2+2e−CH4+4O−2→2H2O+CO2+8e−
The electrons produced in the electrochemical reactions are deposited with the anode and drawn out of the system. At the cathode, electrons are received by oxygen atoms from the air and converted into oxide ions that are transferred to the electrolyte with the reaction O2+4e−→2O−2 and the cycle continues.
Solid oxide fuel cells normally operate at temperatures between about 900° C. and about 1000° C. The operating temperature of a fuel cell, for example, may be influenced by a number of factors primarily related to reaction efficiency. The principal consideration governing the temperature at which a solid oxide fuel cell or any solid-state electrochemical device is operated is the ionic conductivity of the electrolyte membrane. At appropriate temperatures the oxygen ions easily migrate through the crystal lattice of the electrolyte. Therefore, conventional fuel cells must be operated at a high enough temperature to make the ceramic electrolyte sufficiently ionically conductive for the energy producing reactions to occur efficiently. Other operating temperatures may be used to maximize other reactions for use in gas separators or gas generators.
Another consideration in the determination of operation temperature is the type of fuel used and fuel efficiency. For example, methane is plentiful, inexpensive, and rich in hydrogen, the preferred fuel for the cell. Methane and other hydrocarbons may be reformed to produce hydrogen at a temperature of about 650-950° C. Therefore, it is usually desirable to operate a solid oxide fuel cell at least at the lower end of this temperature range. Waste heat from the fuel cell may be used directly in the reformer to pre-treat the fuel before it enters the fuel cell. Waste heat may also be used to generate steam to drive a generator or the like and improve the overall production efficiency of the system.
Unfortunately, most metals are not stable at the high operating temperatures and oxidizing environment of conventional fuel cells and become converted very quickly to brittle oxides. In solid oxide fuel cells, component durability is most problematic at the air electrode where oxidation can take place. In other solid-state electrochemical devices, such as oxygen generators, both electrodes may be in an oxidizing environment during operation of the device, and so both may face this problem. Accordingly, solid-state electrochemical devices have conventionally been constructed of heat-tolerant ceramic materials, such as La1-xSrxMnyO3-δ (1≧X≧0.05) (0.95≦y≦1.15) (“LSM”), and yttria stabilized zirconia (e.g., (ZrO2)0.92(Y2O3)0.08) (“YSZ”). These materials tend to be expensive and still have a limited life in high temperature and high oxidation conditions.
In addition, the thickness of the solid electrolyte, typically hundreds of microns thick, requires an operating temperature above 900° C. to achieve an acceptable ionic conductivity. However, it has also been observed that the resistance of electrolyte layers that are deposited as very thin films is very low at intermediate temperatures ranging between 500° C. and 800° C. Intermediate operational temperatures will allow the use of comparatively less expensive materials to construct the cell and lower the thermal energy requirements.
Several methods exist for forming thin film electrolytes on ceramic substrates, such as sputtering, tape calendaring, sol-gel deposition and physical vapor deposition techniques. For example, plasma spray deposition of molten electrolyte material on porous device substrates has been proposed, however these plasma sprayed layers are still sufficiently thick (reportedly 30-50 microns) to substantially impact electrolyte conductance and therefore device operating temperature. Furthermore, many of these application methods are complex and expensive techniques and the high operating costs as well as the cost of production equipment present a significant barrier to commercialization.
Electrochemical cells with ceramic electrodes and electrolytes in the art have two basic designs: tubular and planar. Tubular designs have traditionally been more easily implemented than planar designs, and thus have been proposed for commercial applications. However, tubular designs provide less power density than planar designs due to the relatively long current path that results in a substantial resistive power loss that is inherent in the design.
Planar fuel cell designs are theoretically more efficient than tubular designs, but are generally recognized as having significant safety and reliability issues due to the complexity of sealing and manifolding a planar stack. To be stacked, the fuel cells require bipolar interconnects adjacent to each electrode that are electrically, but not ionically, conductive. The interconnects allow current generated in the cells to flow between cells and to be collected for use. These interconnects are typically formed into manifolds through which fuel and air may be supplied to the respective electrodes. Due to the highly exothermic combustion resulting from an uncontrolled mixture of hydrogen and oxygen, it is essential that the interconnect manifolds be well sealed at all edges of the planar cell. Moreover, due to required operating temperatures in excess of 900° C. for conventional devices, the interconnect in contact with the air electrode may not be made of metal due to the occurrence of high temperature corrosion.
Prior designs for solid-state electrochemical planar stack devices have used ceramic materials such as lanthanum chromite to form the interconnects. However, lanthanum chromite is a very expensive material, sometimes accounting for as much as 90% of the cost of a device. In addition, it is a relatively brittle material compared to metals and therefore less than ideal for an application requiring an absolute seal. Lanthanum chromite and similar materials are also significantly less conductive than metal, resulting in resistive losses that reduce the overall efficiency of the fuel cell device. These problems have combined to make current planar stack implementations impractical for commercial applications.
Thus, present solid-state electrochemical devices incorporating conventional designs are expensive to manufacture and may suffer from safety, reliability, and/or efficiency drawbacks. Consequently, the cost of electrical energy production from fuel cells is several times higher than the cost of the same electrical production from fossil fuels.
Accordingly, there is a need to provide electrochemical devices such as solid oxide fuel cells that are capable of operating efficiently at lower temperatures and use less expensive materials and production techniques. A method for reducing the cost of materials and manufacturing while increasing the reliability of solid state electrochemical devices would be of great benefit and, for example, might allow for the commercialization of such devices that have been previously too expensive, inefficient or unreliable to exploit. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies in conventional devices.